Study of water sorption of flours (wheat and soy) using a hygrometric method: effect of relative humidity during heat treatment

 The moisture sorption isotherms of wheat and soy flours at 30 °C were constructed using the Novasina hygrometric technique. Wheat flour had a higher sorptive capacity than soy flour, as has previously been demonstrated using a chromatographic sorption technique. Good agreement was found between the Novasina and the static gravimetric methods. The difference between the two methods was of the order of 10.7% for wheat flour and 11.4% for soy flour. Subsequently, the flour samples were heat treated at 100 °C for 1 h and 150 °C for 2 h at various relative humidity environments (0.32<aw<0.90). Sorption data showed a reduced sorptive capacity for both flours for all heating conditions which further decreased with increasing relative humidity values during heat treatment. The decrease in wheat flour sorptive capacity upon heating in the presence of moisture ranged between 11.5% and 55.7% (heating for 1 h at 100 °C) and between 42.1% and 83.1% (heating for 2 h at 150 °C). For soy flour, the decrease in sorptive capacity ranged between 7.8% and 33.8% (heating for 1 h at 100 °C) and between 45.4% and 60.6% (heating for 2 h at 150 °C). Results were correlated with the effect of a specific heat treatment/relative humidity on protein and carbohydrate constituents of the flours.